Fatigue life determination of a damage-tolerant composite airframe

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The methodology proposed in this research extends the current full-scale test approach based on the life factor and the load enhancement factor, and provides information necessary to define inspection intervals for composite structures by studying the effects of extremely improbable, high-energy impact damage. This methodology further extend the current practice during damage-tolerance certification to focus on the most critical damage locations of the structure and interpret the structural and loads details into the most representative repeated load testing in element level to gain information on the residual strength, fatigue sensitivity, inspection methods and inspection intervals during full-scale test substantiation. A reliability approach to determine the inspection intervals to mitigate risks of unexpected failure during the damage tolerance phase, especially with large impact damages, was discussed. This methodology was validated with several full-scale test examples of the Beechcraft Starship forward wings with large impact damages on the front and aft spars. Procedures to generate reliable and economical scatter and load-enhancement factors necessary for a particular structural test by selecting the design details representing the critical areas of the structure is outlined with several examples and case studies. The effects of laminate stacking sequence, test environment, stress ratios, and several design features such as sandwich and bonded joints on the static-strength and fatigue-life shape parameters are discussed with detailed examples. Furthermore, several analytical techniques for obtaining these shape parameters are discussed with examples. Finally, the application of load enhancement factors and life factors for a full-scale test spectrum without adversely affecting the fatigue life and the damage mechanism of the composite structure is discussed. A methodology synthesizing the life factor, load enhancement factor, and damage in composites is proposed to determine the fatigue life of a damage-tolerant composite airframe. This methodology narrows the variability of different aspects of the damaged structure to determine the remaining fatigue life of the structure. In order to prevent unintentional failure of a damaged article during dadt testing, especially when investigating extremely improbable high-energy impact threats that reduce the residual strength of a composite structure to limit load, rigorous inspection intervals are required. The probability of failure of the damaged structure with the enhanced spectrum loads can be evaluated using the proposed cumulative fatigue unreliability model, which was validated through a full-scale test demonstration of a damaged article at the critical load path. Information from this model can be used also to allot economical and reliable inspection intervals during service based on a target reliability and a critical damage threshold. Full-scale dadt test conducted with a visual impact damage on the aft spar (secondary Load path) using the improved lefs based on the design details of Starship forward wing Structure demonstrated the repeated life requirements according the proposed load-life-damage Hybrid approach, and the post-dadt residual strength requirements. The forward-wing dadt test article with a large damage on the front spar (primary load path) demonstrated the capability of the cumulative fatigue unreliability model to predict the damage growth in terms of reliability and the capability of the model to determine the inspection levels. Although it is not a one-to one correlation for the damage propagation or its size, the cumulative fatigue unreliability model highlighted load segments that resulted in gradual progression of local damage, such as possible matrix cracks, and the global impact of high loads that resulted in evident damage growth

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Wichita State University, College of Engineering, Dept. of Aerospace Engineering